Blown amp section on NAD 7020 - What to check now?

TBL

Member
2012-02-13 2:44 pm
I bought two NAD 7020 receivers and like their sound on my JBL 4320, coloured in the highs in a sweet way, very lucid mids, and reasonable bass.

One of the couple is dead as told by the seller, but I found its pre-out working normally, and every preamp section works fine, including the tone amp, all controls, and the phono amp. Its preamp section sounds remarkably hi end like, as if it cost tenfold. Yea.

I have gotten its service manual but as a newbie in ss amp diagnosis, I am scratching my head as to how to approach the issue. BTW, the seller said the receiver stopped working only recently, and its symptom was "no sound", obviously.

On its amp section, the sm advises to check dc offset voltage values. Not much more than that.

Any advice specifically applicable to the NAD 7020 amp section?

Thanks.

Terence
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Checking DC offset (along with supply voltage rails) is probably one of the first tests to be done.

You need to measure the DC voltage at the amplifier output before it reaches the speaker protection relay. Anything higher or lower than a few 10's of millivolts indicates a problem. You could well find one channel output is sitting at the supply voltage level.
 
OK, let's think about this logically and with reference to the 7020 schematic.

If the preamp is working but not the power amps (either of them), we either have two faulty power amps (unlikely but possible) or something common to the power amps but not the preamps (more likely).

Look at the schematic below. The power amps have their own shared secondary, 4A fuses, separate rectifier and bank of 4 2200uF caps, not shared or used by the preamp. Check the 4A fuses and the rails are +/-29V.

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The thermal breakers on each output can fail in their old age, but I would opine that two failing together is unlikely.

Does the preamp mute work on power cycling or not? It is power switch triggered (twin FETs shorting the preamp out) via the negative unregulated power amp rails and from the regulated +29v feed. If the preamp mute/delay isn't working properly, you all most certainly have a PSU issue as described above.
 

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